Appeal to authority is about claiming nothing in the book can be wrong on the basis it came from a respected author. It's not about treating any author as equally good to learn from as expert's just because the content could happen to also be good if you're lucky.

I wonder how that works in other fields. Is it an appeal to authority if one buys things by reading reviews? Does everyone always afford the time or the money to try each thing themselves to decide this is the one they want? Should people never look at the manufacturer brand before making a purchase?

If you know the author is an expert in a certain field, you can use that as a heuristic for deciding whether reading the entire book is worth your time.

How can the content "speak to me" when I am not familiar with a subject? If you want to learn and practice Islam, would you pick a book written by an expert Christian who prefers to denounce it?